A rigid polyurethane foam has an excellent moldability and processability, and is in wide use as a heat insulating material, structural material or shock absorbing material.
In general, a rigid polyurethane foam has closed cells in which such a gas as a volatile chlorofluorocarbon, e.g., trichlorofluoromethane (R-11), or carbon dioxide is enclosed. The gas has a small heat conductivity so that a closed cell rigid polyurethane foam has a high heat insulating performance, and hence is in wide use as a heat insulating material. On the other hand, a rigid polyurethane foam for use as a structural material does not need a high heat insulating performance. Nevertheless, such a volatile chlorofluorocarbon as trichlorofluoromethane is usually used as a blowing agent for the production of a rigid polyurethane foam for use as a structural material since it is inexpensive and easy to handle.
The conventional chlorofluorocarbons exemplified by trichlorofluoromethane are chemically stable so that they diffuse into the stratosphere to destroy the ozone layer. As a result, the solar ultraviolet radiation is not absorbed by the ozone layer, but it reaches the surface of the earth, and is causing a global environmental problem. For this reason, the use of trichlorofluoromethane has been limited since 1989, and the use of trichlorofluoromethane for the production of polyurethane foam as well.
It is of course possible to use a closed cell rigid foam as a structural rigid foam. However, as set forth above, the closed cell foam is produced while it contains a gas in the cells, so that it is necessary to employ a jig having a strength sufficient to withstand a large foaming pressure when the foam is produced. Moreover, the closed cell foam has not a sufficient dimensional stability, in particular, a thermal dimensional stability.
Therefore, there is no need to use a closed cell rigid foam as a structural material where a high heat insulating performance is not necessary. Nevertheless, a closed cell rigid foam has been in fact used as a structural material on account of the difficulty of producing an open cell rigid polyurethane foam having a relatively high mechanical strength without the use of chlorofluorocarbons or freons as a blowing agent. When water is used as a blowing agent, a closed cell rigid polyurethane foam is readily produced since an organic polyisocyanate compound reacts with water to form urea bonds to crosslink the molecular chains while producing carbon dioxide.
Under these circumstances, there have been proposed a variety of methods in which a cell opening agent is used, as disclosed in, for example, Japanese Patent Application Laid-open No. 63-89519 or No. 61-51021. According to the method, an open cell foam is obtained by use of trichlorofluoromethane as a blowing agent, however, the trichlorofluoromethane is released into the air when a foam is produced, with a result that such a gas might cause an environmental contamination. In addition, such release of gas is disadvantageous from the economical standpoint.
In the meantime, the use of a shock absorbing material has started in recent years as one of the safety measures for motor driving to absorb a shock when an automobile collision occurs. It is already known that a rigid polyurethane foam having open cells and a large cell size is suitably used as such a shock absorbing material, as disclosed in, for example, Japanese Patent Publication No. 54-4027 or No. 52-34678. Thus, the use of an open cell rigid polyurethane foam as a shock absorbing material has become important in recent years among the many uses of structural materials.
As an open cell rigid polyurethane foam for use as a shock absorbing material needs no high heat insulating performance, it is desirable not to use a chlorofluorocarbon as a blowing agent. However, as set forth hereinbefore, it is usually difficult to produce an open cell rigid polyurethane foam without the use of chlorofluorocarbon as a blowing agent, still more a foam for use as a shock absorbing material.
In general, a shock absorbing material is used to absorb and relieve a shock, and accordingly it is important that a shock absorbing material has a high effective compressibility. FIG. 1 is a simplified graph to illustrate the relationship between compressibility and compressive stress of a rigid polyurethane foam under a static compression test, in which the line A-B indicates a compressive strength, and the compressibility at a point B indicates an effective compressibility. The amount of absorbed energy by a shock absorbing material is shown by a shaded region in FIG. 1. It is necessary that a shock absorbing material has a thickness as small as possible if resident space should be large, as the case with an automobile, and therefore, it is necessary that a shock absorbing material has a large effective compressibility as well as a large compressive strength.
The point B appears around a compressibility of 30-50% with regard to a usual rigid polyurethane foam. However, it is desirable that a rigid polyurethane foam has an effective compressibility of at least 70%, and preferably at least 80%, so that it is suitably used as a shock absorbing material. It is also necessary that a rigid polyurethane foam breaks into pieces, preferably into powder, when a shock is given thereto so that the foam has a high effective compressibility, as disclosed in, for example, Japanese Patent Publication No. 54-4027.